In recent years, there has been an increase in demand for shortening the delivery time of molds, and thus, a highly efficient process for preparing molds has been desired. For the high-efficiency roughing of molds, ball end mills and radius end mills are widely in use. However, in a ball end mill, because the cutting amount in the vicinity of the central part becomes large, cutting rate does not increase, leading to inferior machinability. In addition, there is a disadvantage in that the chip pocket in the vicinity of the central part is small, and chip removal is poor. Therefore, recently, radius end mills are used more often for the high-efficiency roughing of molds. By using a radius end mill, because the cutting amount at the outer periphery becomes larger, the chip pocket becomes wider. Thus, chip removability becomes good, and cutting is performed at high-speed cutting part of the end mill, enabling highly efficient roughing, compared to the ball end mill.
However, lately, with the use of new molding materials and request for enhanced molding efficiency, demands regarding the durability of molds are increasing. Therefore, there is a growing interest in the adoption of high-hardness mold materials, which show much higher hardness than regular mold materials. In the high-efficiency roughing of such high-hardness mold materials, there is a problem in that fractures tend to occur, and that abrasion progress more quickly, even with the use of conventional radius end mills. Furthermore, vibration is more likely to occur during cutting, particularly when machining corners of the mold, and so far, adequate service life and stability has not yet been attained.
In order to suppress end mill abrasion during cutting of such high-hardness materials, to enhance fracture resistance, and to suppress vibration during cutting, to thereby achieve highly efficient and stable machining, several propositions, including a radius end mill, have been made.
In Patent Document 1, a face mill, in which the cutting part of the bottom edge is arc-shaped, arching along at least part of the length of the tool, when seen from the tool end, is disclosed, so as to enhance the metal removal rate and suppress tool abrasion, to thereby lengthen the tool life.
In Patent Document 2, an ellipsoidal end mill, wherein an approximately partial ellipsoidal cutting edge is formed at the tip of a tool body that rotates around a shaft center, and the focus of said ellipse is in an approximately symmetrical position from the shaft center, and at least part of said cutting edge is composed of a super-hard material, is disclosed.
In Patent Document 3, a face mill for the purpose of high feed rate machining and high-efficiency machining, wherein the cutting edge of the end mill is composed of a peripheral edge, an arc-shaped bottom edge, and an arc-shaped corner R edge that connects them, is disclosed.
In Patent Document 4, a cemented carbide solid end mill comprising a plurality of corner edges, wherein said corner edge is composed of a main corner edge 1 that forms the rotational trajectory of said end mill and a sub-corner edge 2 that is undersized compared to said rotational trajectory, wherein said main corner edge is arc-shaped, said sub-corner edge is of the same or smaller arc-shape than said main corner edge and 0.5% to 10% undersized than the cutting diameter forming said rotational trajectory, is disclosed.